A helicopter is known which can carry out the vertically rising and falling of the fuselage, the forward movement and backward movement, and the aerial stop, by rotating a main rotor. When the helicopter has N main rotor blades (N is a natural number), the N/Rev vibration is caused due to the rotation of the main rotor. Moreover, in the helicopter, when the helicopter receives a wind or when the helicopter turns over suddenly, there is a case that the number of rotations of the main rotor changes so that the frequency of the N/Rev vibration changes. In the helicopter, good habitability is desired and it is desired to reduce the vibration which is propagated to passengers of the helicopter.
A vibration reducing apparatus is known which is installed in the helicopter to reduce the vibration which is propagated to the passengers of the helicopter. It is required that such a vibration reducing apparatus is more compact.
The vibration reducing apparatus is disclosed in JP 2011-169382A (Patent Literature 1) which reduces the vibration of a vibration damping object more surely. The vibration reducing apparatus includes an elastic body and a dynamic mass supported by the vibration damping object through the elastic body, a control mass and an actuator for moving the control mass to the dynamic mass. Such a vibration reducing apparatus moves the control mass appropriately to the dynamic mass, so that the frequency and amplitude of vibration of the dynamic mass to the vibration damping object can be changed and the vibration of the vibration damping object can be reduced more surely.
A vibration reducing apparatus is disclosed in JP 2009-138893A (Patent Literature 2) in which energy loss is small and vibration damping of a vibration body is possible. The vibration reducing apparatus includes a measuring unit provided for a structure and configured to measure the frequency and amplitude of vibration of the structure in up and down directions, a vibration damping unit provided for a pillar or a stake of the structure and configured to generate vibration damping force in the up and down directions, and a control unit configured to control the magnitude of the vibration damping force based on the frequency and amplitude of vibration of the structure measured by the measuring unit. In the vibration reducing apparatus, the vibration damping unit includes a vibration applying unit vibrating in up and down directions, and a variable stiffness spring provided between the vibration applying unit and the pillar or the stake. The control unit drives the vibration applying unit to vibrate based on the amplitude of vibration, and changes the stiffness of the variable stiffness spring based on the frequency.
The technique to reduce vibration through movement a liquid sinker by using an actuator having a piezo device is disclosed in literature “Results from the dynamically tailored airframe structures program”